How able are you to modify a weapon after it has been created?
When it comes to magical abilities that use up the enhancement bonus, you are only able to ADD to the existing item. You are unable to replace them (except in rare cases like upgrading flaming to flaming burst). So you would be unable to have a flaming weapon, become a shocking weapon instead.
The above example doesnt change the weapons purpose or materials. Altering Existing Magic Items (scroll up)
The standard rules don't allow item creation feats to alter the physical nature of an item, its default size, its shape, or its magical properties. For example, there is no mechanism for using crafting feats to change a steel + 1 longsword into an adamantine + 1 longsword , a Large + 1 chain shirt into a Medium + 1 chain shirt , boots of speed into an amulet of speed , or a + 1 unholy longsword into a + 1 flaming shock longsword. Many GMs might decide that these kinds of transformations are impossible, beyond the scope of mortals, or not as cost-efficient as crafting a new item from scratch. Others might allow these sorts of transformations for free or a small surcharge. Keep in mind the following warnings.
So my question is, what about changing a base weapon in a reasonable way? The best example I can use is the longbow, composite. Would you be able to upgrade your +1 longbow, composite (+1) to a +1 longbow, composite (+2)? Is this allowed, or is the area so grey that its a GM decision?